It is imperative that serious Staphylococcus aureus diseases such as bacteremia and infectious endocarditis are treated aggressively with effective antimicrobial agents. These diseases had mortality rates of 80 and 100 percent respectively in the pre-antibiotic era. Most hospital strains of S. aureus are now methicillin-resistant (MRSA), and such strains are typically resistant to multiple other antibiotics. The glycopeptide antibiotic vancomycin was the sole remaining antibiotic to which S. aureus remained uniformly susceptible. Recently. vancomycin-resistant strains have arisen in patients during long-term vancomycin therapy (so called glycopeptide-intermediate susceptible S. aureus or GISA strains). It is imperative that we understand the mechanism of vancomycin resistance in such strains. Vancomycin-resistant strains have been step selected in my laboratory. It is likely that the resistance mechanism involves several mutations and is multifaceted. I propose to study the mechanism of S. aureus vancomycin resistance in laboratory and clinical strains. I will study the composition and structure of peptidoglycan, and teichoic acid and lipoteichoic acid in GISA strains, and attempt to understand the mechanism of decreased autolytic activity observed in such strains. Cell wall alterations appear to be involved in vancomycin resistance. I will attempt to understand the increased NaC1-sensitivitv of GISA strains through studying the accumulation of compatible solutes. and whether Na+ ions accumulate intracellularly to growth inhibitory levels upon NaCl stress. Limited studies toward understanding the genetic basis of vancomycin will be undertaken. Nucleotide sequence analysis of cloned selected genes that physiological studies indicate may be altered in GISA strains will be carried out. It is expected that these studies will lead to improved methods for the control of methicillin-resistant and vancomycin-resistance S. aureus infections, and form the basis for development of novel antistaphylococcal agent.